Camarasaurus in rear (left) and side (right) views. Com-
pare with figure 4.7, which shows the chevron bones from diplodocid sauropods.
of the skeleton was preserved and most of the bones were still articulated (fig. 4.18). The small Cama- rasaurus must quickly have become buried follow- ing its premature death, for the remains show little sign of scavenging, decomposition, or postmortem transportation of the bones in this extraordinary specimen. The fossils at Dinosaur National Mon- ument are preserved in a lens of sandstone in the Brushy Basin Member that accumulated as a sandbar in a river channel. The sandstone body, about 15 feet thick, has at least three bone-producing horizons (Lawton 1977). A. R. Fiorillo (1994) has suggested that the bones in these horizons were buried over a relatively short period, perhaps only a few months. The remarkable specimen of the juvenile Cama- rasaurus from Dinosaur National Monument prob- ably arose from the nearly immediate burial of the carcass soon after the death of the baby sauropod. The young camarasaur must have died very close to or perhaps even on the sandbar in the river channel.
Camarasaurus obviously was adapted to feed on different kinds of plants than those that sus- tained the diplodocids. The robust teeth would no
doubt have been able to withstand powerful crush- ing forces that would have snapped the delicate teeth of Diplodocus. A. Fiorillo (1991) microscopi- cally examined the wear surfaces of Camarasaurus teeth and found coarse scratch marks and pits, indi- cating that relatively tough plant material was pro- cessed by them. Unlike the diplodocids, which used their teeth primarily for raking vegetation into the mouth or stripping it from branches, Camarasaurus probably chewed leafy food more thoroughly before passing it through the digestive tract. Although the neck was relatively short, it was flexible. Cama- rasaurus could probably have lifted its head a little higher than was comfortable for the diplodocids. Given the differences in their teeth, body size, and posture, it is highly doubtful that Camarasaurus competed with the diplodocids for the same food resources. Camarasaurus more likely was a high browser and fed by nipping off the fronds of tree ferns or the leaves of cycads that grew well above the ground (fig. 4.19). This arborescent vegetation may have been tougher and more fibrous than the lower-growing shrubs that provided the bulk of the diplodocid diet. Based on the overall abundance and geographic distribution of Camarasaurus and the diplodocid sauropods in the Morrison Forma- tion, they certainly appear to have been compatible companions who romped together across the broad alluvial plain.
There seems to have been considerable varia- tion among Camarasaurus during the late Jurassic. At least three species of Camarasaurus populated the Morrison habitat in western North America: C. supremus, C. grandis, and C. lentus. Remains of C. supremus were first found near Garden Park, Col- orado, and indicate a massive animal with a femur nearly 6 feet (2 meters) long. C. lentus is extremely common at Dinosaur National Monument and was very similar to C. supremus except for its smaller size. C. grandis is also fairly common in Utah and differs from the other two species in several ana- tomical details of the vertebrae from the back- bone. A fourth species, C. lewisi, was described by 4.18. Head and neck of the remarkably complete skele-
ton of a juvenile Camarasaurus from Dinosaur National Monument.
McIntosh and others (1996) on the basis of unique fossils that were originally thought to indicate a sep- arate genus, Cathetosaurus.
Family Brachiosauridae
The family Brachiosauridae includes some of the most immense land animals that ever lived on the earth. These gigantic sauropods had relatively long skulls, not unlike those of the diplodocids, but with large bulging nostrils placed high on the face, a feature somewhat reminiscent of the camarasau- rids (fig. 4.20). The teeth of the brachiosaurids were large, lining the entire margin of the jaws, and had a unique chisel-like form. The thirteen cervical verte- brae were highly elongated, giving the brachiosau- rids the longest necks of any dinosaur. The neural spines of the cervical vertebrae were not bifurcated; each vertebra possessed very long ribs that extended along the lower neck, probably to anchor large neck muscles and ligaments. The tails, in contrast to the enormous necks, were relatively short and strongly tapered.
Perhaps the most distinctive feature of the bra- chiosaurids was the unusual proportions of their limbs. The forelimbs were significantly longer than the hind limbs, giving the relatively short back
a pronounced upward tilt from the hips to the shoulders. This limb structure, coupled with the extremely long necks, made the brachiosaurids look much like overgrown reptilian giraffes (figs. 4.21 and 4.22). Because of this unique form, the forelimb of the brachiosaurids carried much more weight than those of other sauropods. Consequently the front legs are massive and columnar, proportioned much like the forelimbs of an elephant. The shoulder gir- dle of brachiosaurids, which transmitted the weight of the body to the elephantine forelimbs, was more massive than in most other sauropods. In addition, the front feet are well designed to support the enor- mous load that they carried. The toes were formed from massive bones and were widely spread to dis- tribute the immense weight, which may have totaled as much as 50 tons. The middle foot bones (meta- carpals) were very long and positioned almost ver- tically to resist the crushing forces applied to them. Thick pads of shock-absorbing cartilage were pres- ent in the soles of the feet and between the limb bones to cushion the pounding that would have resulted from the movement of such large beasts. The brachiosaurids were truly ponderous creatures; the largest of them would have made Camarasaurus look runty in comparison.
In general brachiosaurids are very uncommon in the Morrison Formation of western North Amer- ica. No brachiosaurids remains have yet been rec- ognized among the fossils collected from Utah exposures of the Morrison Formation, but they are known from localities in western Colorado. The Grand Valley and Uncompahgre Plateau regions of Colorado, only 30 miles or so from the Utah bor- der, are among the few places in North America where brachiosaurid remains have been discov- ered. Moreover, several different types of brachio- saurids are represented among the Morrison fossils excavated from that region. Brachiosaurus altithorax was first identified from Morrison exposures near Grand Junction, Colorado, in the early 1900s (Riggs 1903, 1904). In more recent years several other bra- chiosaurids have been uncovered at the nearby Dry 4.20. Skull of Brachiosaurus. Note the very large nos-
tril (en) located high on the skull. The smaller open- ing below the nostril is the antorbital fenestra (aof). The teeth are large and chisel-shaped in the brachiosaurid sauropods.
Mesa Quarry in the Uncompahgre Plateau by scien- tists from Brigham Young University (Jensen 1985a,
1985b; Miller and others 1991). Included in the more recent discoveries is the enormous brachiosau- rid originally named Ultrasaurus but later renamed Ultrasauros. The bones on which the identification of Ultrasauros is based indicate a very large animal, exceeding 100 feet (33 meters) in length and weigh- ing more than 125 tons! Although most paleontol- ogists now consider Ultrasauros an invalid genus because the bones that define it appear to be a mix- ture of more than one dinosaur, it still provides evi- dence of extremely large brachiosaurids in the Utah region. Even the better-known Brachiosaurus was considerably larger than most of its sauropod con- temporaries, reaching a length of around 75 feet (25 meters). The long necks of the brachiosaurids, coupled with their giraffelike stance, would have allowed them to raise their heads 45 feet or more above the ground.
The unique anatomy of the brachiosaurid sau- ropods seems to suggest that these sauropods were especially well adapted to the niche of a high browser. No other dinosaur could have competed with them for the foliage crowning the tallest of the Jurassic trees. The brachiosaurids were clearly very successful in this ecologic niche, for their remains have been found in North America, east Africa, and 4.21. The skeleton of Brachiosaurus. The
long forelimbs lifted the shoulders well above the hips, giving this sauropod its unique giraffelike appearance. The long neck angled upward, allowing the brachio- saurids to feed on vegetation some 50 feet above the ground. Based on reconstruc- tion of Norman 1991.